Because of recent growing concern on environmental pollution problems, there are eagerly desired thermal power generation systems or engines which can exhibit high combustion efficiency and automotive engines which can clear recent severe regulations on exhaust gases. Measures taken for these purposes result in an combustion gas having a higher temperature and, in consequence, adjacent members, e.g. a system for purifying an exhaust gas, connected to the thermal power generation engines or automotive engines are necessarily encountered to the higher temperature. Accordingly, further enhanced heat resistance is required for them. For heat resistance of a material, high temperature oxidation resistance, that is a property of the material that it is well durable under an atmosphere of a high temperature gas, is required, in addition to high temperature strength of the material.
In order that a material is excellent in high temperature oxidation resistance for a purpose of the invention, it is desirable that the material does not undergo abnormal oxidation and exhibits the smallest possible oxidation weight gain during its service. Besides, the material should be excellent in adhesion of oxide scale (oxide film). Since operation of internal-combustion engines such as automobile engines includes repeated starting and stopping of driving and thermal power generation systems are operated in daily start and stop (DSS) fashion, heat-resisting members connected to such engines of systems are subjected to repeated cycles of heating and cooling. Accordingly, with materials poor in adhesion of oxide film, it spalls posing problems of clogging of piping and reduction in thickness of the heat-resisting members themselves which might lead to mechanical breakage of the members.
Austenitic stainless steels have better high temperature strength than ferritic stainless steels. However, because of their larger thermal expansion and in turn larger thermal strain, when compared with ferritic stainless steels, austenitic stainless steels are liable to cause cracking due to thermal fatigue when subjected to repeated cycles of heating and cooling. Moreover, because of their large difference between thermal expansion of the base metal and that of oxide scale, austenitic stainless steels exhibit an increased amount of scale spalling.
For these reasons ferritic stainless steels have been used in the manufacture of automobile exhaust gas systems. For example, a ferritic stainless steel, SUS430JIL has been used in the manufacture of an automobile exhaust manifold. This steel, however, poses problems of an increased amount of scale spalling and of an expensive cost of the material.
U.S. Pat. No. 4,640,722 discloses and claims a ferritic stainless steel exhibiting improved cyclic oxidation resistance and creep strength, after a final anneal at 1010.degree. to 1120.degree. C., which develops Nb-Si rich Laves phases, suitable for use in the manufacture of automobile exhaust gas systems, consisting essentially of, in weight percent, C.ltoreq.0.05%, Mn.ltoreq.2%, 1.0% &lt;Si.ltoreq.2.25%, Al&lt;0.5%, 3.times.Al.ltoreq.Si, 6%.ltoreq.Cr.ltoreq.25%, Mo.ltoreq.5%, 8%.ltoreq.Mo+Cr, N.ltoreq.0.05% 4.times.C+3.5.times.N.ltoreq.at least one of Ti, Zr and Ta.ltoreq.0.5%, total Nb.ltoreq.0.3%, 0.1%.ltoreq.uncombined Nb, and balance essentially Fe. However, this U.S. Patent does not teach how to suppress oxide film spalling nor address to low temperature toughness and fabricability of the steel. For use in the manufacture of automobile exhaust manifold, the steel is required to have improved adhesion of oxide film and low temperature toughness in addition to improved cyclic oxidation resistance.
U.S. Pat. No. 4,461,811 discloses and claims a stabilized ferritic stainless steel consisting essentially of, in weight percent, C.ltoreq.0.03%, N.ltoreq.0,05%, 10.5%.ltoreq.Cr.ltoreq.13.5%, Al.ltoreq.0.10%, Ti.ltoreq.0.12%, Al+Ti.ltoreq.0.12%, the sum of Ti and at least one of Nb and Ta in an amount sufficient to stabilize C and N, balance essentially Fe and impurities. It is taught that the stabilized steel is wettable by a brazing filler such as Cu and Ni and thus suitable for use in the manufacture of brazed parts which are composed in heat exchangers and exhaust gas systems requiring oxidation and corrosion resistances at elevated temperatures inherent to ferritic stainless steels. However, it is not clear whether improved adhesion of oxide film, low temperature toughness and fabricability are concurrently possessed or not by the stabilized steel according to the '811 patent. No measure to achieve these improved properties is not suggested nor recognized.
U.S. Pat. No. 4,417,921 discloses a ferritic stainless steel consisting essentially of, in weight percent, C.ltoreq.0.03%, N.ltoreq.0.03%, C+N.ltoreq.0.04%, 11.5%.ltoreq.Cr.ltoreq.13.5%, Mn.ltoreq.1.0%, Si.ltoreq.1.0%, Ni.ltoreq.0.5%, Cu.ltoreq.0.15%, Ni+3.times.Cu.ltoreq.0.80%, at least one of Ti and Nb in an amount from 0.1% and 4 .times. (C + N) up to 0.75% balance essentially Fe and usual steelmaking residuals. It is said that this steel is suitable for integrally-finned tubing of heat exchangers because of its excellent weldability, ductility, fabricability and resistance to stress corrosion cracking. The '921 patent, however, does not teach effects of alloying elements on high temperature properties of ferritic stainless steels, particularly on high temperature oxidation resistance and adhesion of oxide film. It does not address to properties required for automobile exhaust gas manifolds.
For use in exhaust gas systems, particularly automobile exhaust gas manifolds, eagerly desired are inexpensive ferritic stainless steels which have high temperature strength well comparable to SUS430JIL and which are excellent in high temperature properties including high temperature oxidation resistance and adhesion of oxide film and which are excellent in low temperature toughness and fabricability. This desire is growing in view of recent progress in combustion efficiency of internal engines and improvement in exhaust gas purification. An object of the invention is to provide a ferritic stainless steel satisfying the above-mentioned desire in the art.